The "writeconf" method (backend/nagios/systemapi.py) of the Nagios module restarts the Nagios service after writing configuration files.
For the Ceph module we have two special methods "write_nagios_configs" and "remove_nagios_configs" (backend/ceph/systemapi.py). These methods doesn't restart the Nagios service yet. So after creating the Ceph configuration files, no .rrd files will be created for the clusters.
Wrap the restart of the Nagios service in backend/nagios/systemapi.py by its own DBus method and call this method from the Ceph module.